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CETME headspace/gap...kinda long...input appriciated

So on my rifle, I had a gap of about .002”-.003”, bought some +4 rollers that brought it up to .006”... within "spec". Measured via slamming the bolt home to a closed chamber, and dry firing (dry firing makes no difference on gap for my rifle btw). But If I use a round, a sliver bear one, and let the bolt slam home, I get a gap around .009-.010. The HK manual doesn't specify if gap is checked empty or loaded or dummy loaded.

Below I’m explaining how I see this works… I make some assumptions, and it seems to make sense to me, but don’t “use” this to determine if its safe to shoot because I’m not 100% sure of what I’m saying is so (thus the post and questions)

Most diagrams I seen, show, or seem to show, the bolt resting on the barrel when closed with a round in the gun. My rifle clearly does not, or the round I'm using is out of spec, BUT it seems, to my experience most gun’s bolts do not rest directly against the barrel, my CETME is the same way. Too me it seems more logical to check gap with a round in it, as that is how it will be set up when it is fired, not empty. As from my understanding the gap is a measurement of how the bolt is locking up, too little gap, too much lock up; too much gap, very little lock up. From what I have researched, low bolt gap will lead to cycling problems in the extraction area, where too much and it won't lock up, causing a violent rearward movement of the bolt and burst casings.

A very slight movement of the bolt translates to a larger movement of the bolt carrier in these guns (thus how the mechanism works), so therefore my bolt is very close, but not touching the barrel, since gap changed only .004 with the bolt resting against the barrel vs with a round. I read the lever ratio is about 1:4 between the bold and carrier, so my bolt sits about .001" from the barrel with a round in it. I would understand that this would give you and idea how far the bolt is from the barrel, and thus how much of the cartridge case is supported or unsupported. I believe headspacing and gap are different things, but related to each other. I think to get a real accurate measurement of “headspacing” by which I mean the distance from bolt to barrel, you would have to measure with out a bullet, and then with, and take the difference and divide by 4.

But I don’t know what the safe range is exactly. Just guessing looking at headspace gauge lengths: 308 win. GO-1.630” and NO-GO 1.634” (Field Reject 1.638”) the difference of .004” (or .008” with F.R.) which, assuming GO means the bolt is touching the barrel or nearly so, that means it can have .004” distance from the barrel to bolt, or unsupported case. For 7.62x51: GO 1.635, Field Reject is 1.6455” difference of .0105” much more than 308, and therefore why the 7.62 have thicker brass. I’m assuming all rifles gaps change with a round chambered, which I don’t know if that is true, also my extractor does not extend past the bolt face, I have heard that some do?

(by my assumptions) This would mean that, for example, that as long as your change in bolt gap is less than .012, then your headspace is in range for 308 (bolt-barrel distance would be 0.000-.004”), if change is less than .02”, but total gap is less than .02” (well .0105*4=.0420” but total bolt gap needs to be less than .02 for proper and safe operation) then it would be in range for 7.62. If there is no change in bolt gap (but still have bolt gap) then you would have a space between the barrel and bolt of 0, meaning the bolt is resting on the barrel, so either the round is short, or the chamber is long (or extractor extends form the bolt face, good I would think if it doesn’t extend more than .004”… I think). If no change in gap, and no gap, then something is worn out of spec. Remembering you still want you gap to be in the .004-.020 range for proper and safe operation of the locking mechanism.

To me, though, thinking about it, IF im right, then this system has a fairly large room for different length or cartridges, basically making the rifle “automatic headspace adjustment” within a range, though small, always locking the cartridge in tight. Even though the 308 and 7.62 have different GO gauges length, the CETME system will adjust to it, and could be checked as I said above.

This would also play into the ground bolt not being safe, as you may get a good headspace (change in gap) the parts may not be locking up tight, allowing play in the system which could, when under pressure, cause the headspace to increase rapidly leaving too much casing exposed, initiating self-disassembly.

I'm mostly curious about this as I read about 7.62x51 vs 308 in guns, one of the concerns being the headspace is different between the two, the Nato chamber can have more headspace than a 308 before it is no longer safe. It seems a lot of this is discussed in respect to typical operating guns (ie M14, FN FAL) but the CETME/HK are different in this respect as they can, in a way, vary the headspace they have depending on the cartridge. I’m trying to better understand how headspacing on CETMEs works.

Maybe... this may be asking too much, those out there with cetme's particularly ones on the edge of gap tolerance, tell me what their gap is without a round, and (if you feel comfortable and safely... maybe do it at the range) see what your gap is with a loaded round or better yet a dummy round? Also if your extractor extends past the bolt face and if so by how much. I'm curious if what my rifle does is normal or not, and what it does to gap, if anything.

Ugh... if you read all of this kudos to you... lol my head is starting to hurt... I'm not 100% about my speculations, so correct/debate me on any point I've made... plus I’m a bit tired... I want to understand this system more.

With a round in the chamber the bolt face will NOT touch the barrel face.... hence the change in gap, and this is why all the manuals (along with the safety aspect) tell you to measure gap on an empty chamber.

Yours is no different than anyone else's... in fact, if the gap DIDN'T change, then there would be a problem with a chamber cut too large.

also, after skimming the rest of the post..... you've pointed out specifically how/why one rifle will run like a top with a .005 gap, and another will not run with that gap, but run's great with a .018 gap...... with so many different manufacturers of barrels out there, there will be some variation.

Everything I read sounds logical and straightforward though. All your measurements seem solid. Only thing I have against it is that you should NOT measure gap with a round in the chamber because differences in tolerances for rounds are MUCH higher than tolerances in the chamber/bolt area. They designed the rifle to be measured in it most consistent state..... boltclosed, empty chamber, hammer dropped to hold tension. This is the only consistent time during the action, and the only time that a truly consistent and accurate measurement can be taken. Some guys will have to tweak gap here and there to get things cycling right, most don't, but all measurements and differences should be calculated on an empty chamber.

OK, headspace on any rimless shouldered cartridge rifle is a measurement of the length of the chamber from a predetermined point on the sholuder to the face of the bolt.

This is done to ensure that the cartridge will be adequately supported in the chamber, and will not rupture from the internal pressures.

With all bolt-action and most other semi-auto action guns this check is performed with guages specifically manufactured to the exact length of what is the minimum length of the chamber, and the accepted maximum safe length. GO/NO-GO.I am not going to go into FIELD guages here, as they work on the same principal as the NO-GO.

These guages are used because there is no reliable method to measure this space in the chamber with the bolt closed.

On these rifles the length of the chamber is affected by the wear on the inside of the chamber itself, (minimal, but present), wear on the bolt face itself, (also minimal but present), wear on the locking lugs of the bolt, (signifigant), and wear on the locking sholders of the receiver/trunion, (also signifigant). Because of the static orentation of the bolt face to the locking lugs on the bolt, wear in the area of the locking lugs/shoulders directly reflects as a change in the length of the headspace when the bolt is closed because the bolt face becomes positioned further rearward in relation to the shoulder of the chamber due to the wear.

With the Roller locked delay blowback system, there are some similarities to the bolt systems mentioned above. There is a chamber, and a bolt face, and locking shoulders in the receiver/trunion, but the way that they are different is that there are no static locking lugs attached to the bolt. This means that as the locking shoulders and the rollers wear, (remember, this is where all of the signifigant wear occurs),the rollers move to compensate for the change, thus keeping the required pressure on the bolt face to support the cartridge provided you still have bolt gap present.

How does this relate to bolt gap?

The bolt gap is like your headspace guages, it tells you how much wear the parts in the rifle that get worn out have experienced. Only it tells you in much more detail.

Bolt gap is a relative thing, and it will differ from one rifle to the next. Suffice it to say that the gap is set at the factory, (or wherever the barrel and trunion are assembled), to a safe distance, usually as close to .020" as possible without going over. This is done with an empty chamber. After that all measurements to verify the degree of wear are worthless if they are not performed under exactly the same conditions, (empty chamber), because these measurements are to be made in relation to the original one.

If the designers had a special plug that was used for this measurement, then we surely would need it in order to properly measure the bolt gap on our firles. But the designers were manufacturing BATTLE RIFLES, that would need to be maintained in the field, possibly without access to specialized tools, so they opted for the most economical, simple method available:

Realisticly, all you need for guages to measure the gap is one .004" and one .020".

If the .004" guage cannot be inserted, there is a problem. Likewise if the .020" can be inserted, there is a problem. Otherwise, if the .004" guage can be inserted and the .020" one cannot, then all is good to go.

Last edited by rustypirate; 11-27-2008 at 12:07 PM.

Some people are like a slinky...Not really good for anything, but fun to push down the stairs.

thanks for the input. I see what you are saying about the need for checking bolt gap consistently from the factory to the field. That makes sense. I read on another post here about someone who changed their extractor on the cetme, and that got them more gap, as the extractor sat just a bit forward of the bolt face. Now my questions is this supposed to be how it is? If so I would imagine then, that the extractor itself acts like a gauge, spacing the bolt face from the barrel at some predetermined point. If so how far is it supposed to sit out from the bolt face? Mine it sits smooth with the bolt face so if it was supposed to sit farther out, then a new extractor would give me more gap, and would make the change from empty to a round in the chamber less. Or is the extractor supposed to sit flush with the bolt face?

If an extractor is adding/subtracting bolt gap... something is not right.

In order for that to happen, the extractor would be taking the brunt force of the action slamming home on the barrel face..... Basically it wouldn't last long. As Rusty said, the extractor should be flush or slightly recessed.